1. Field of the Invention
This invention relates to a method of manufacturing liquid crystal display devices, particularly the alignment layers thereof, and to the liquid crystal display devices and alignment layers prepared by said method.
2. Description of the Related Art
Liquid crystal display (LCD) devices are well known. Optoelectronic devices of this kind generally comprise a cell in which a liquid crystal composition is housed in a cell spaced between two polymeric alignment layers, each supported on an optically transparent electrode which in turn is supported on a transparent plate; e.g., a glass plate.
While a number of resins and resin systems may be employed as the alignment layer (for example, polyvinyl alcohol, acrylate esters, cyanoacrylates, thermosetting unsaturated polyesters, celluloses, polyamides, polyimides and the like), the most commonly used polymer for the alignment layer is a polyimide. Generally, such alignment layers are formed by applying a solution of the polymer resin in a solvent to the substrate electrode. Thereafter, the solvent is removed (e.g., by evaporation), and the coating baked for several hours to complete the removal of the solvent and formation of the resin film. Thereafter, the surface of the alignment layer is then conventionally rubbed utilizing a flannel cloth or buffing wheel so as to generate the alignment characteristic of the resin. In the liquid crystalline display device, the rubbed surface of the alignment layer causes the liquid crystal molecules to adopt a small angle relative to the surface of the alignment layer. This "tilt angle" is usually of the order of 1.degree.-3.degree..
In the preparation of a polyimide alignment layer, for example, a solution of the polyimide resin in an organic solvent (e.g., 16% polyimide in N-methyl pyrrolidone) is applied by spin-coating onto the surface of the electrode. The solvent is removed by evaporation and the coating allowed to dry on the surface. Usually, the coating is baked at high temperatures (e.g., 200.degree.-300.degree. C.) for several hours to complete the removal of the solvent and the formation of the resin film. This process is usually operated as a bath-wise process because of the need to wait for curing of the alignment layer. Finally, the cooled resin alignment layer is then rubbed unidirectionally to provide the alignment characteristic to the layer. The resultant alignment layer generally will provide a "tilt angle" in liquid crystalline molecules on the order of 2.degree.-3.degree..
While tilt angles on the order of 1.degree.-3.degree. are acceptable for general use, special high tilt angles are required for the more advanced, special high-tech liquid crystalline display devices employing the helix or supertwist and chiral technologies, especially the 270.degree. Supertwist Birefringence Effect technologies. Such technologies generally need tilt angles on the order of 5.degree.-14.degree., preferably 7.degree.-14.degree. and higher. Various methods are being sought and suggested for enhancing the tilt angle, especially with respect to polyimide alignment layers. Such methods are taught in "Surface Alignment of Liquid Crystals by Rubbed Polymer Layers" by A. Mosley et al., DISPLAYS, January 1987, p. 17. Additionally, improved tilt angles have also been achieved through the development of specialized electronic grade polyimides; however, such polyimides are very costly, on the order of about $1,000/kilogram for a 15% solution. Furthermore, because of the inherent waste associated with spin-coating, the cost of manufacturing such liquid crystalline display devices becomes prohibitively expensive.
In addition to specialized polyimides, others have developed or identified resin systems suitable for use as liquid crystalline alignment layers which provide enhanced tilt angles. For example, in European Patent Publication EP 0,177,271, Imperial Chemical Industries PLC describes liquid crystalline alignment layers comprising a polyphenylene polymer. Such alignment layers are applied by spin coating from a solution and result in tilt bias angles of between 5.degree.-15.degree..
While the art has identified new methods and materials for the development of improved liquid crystalline alignment layers, generally all of such methods and materials still employ a spin-coating process. Although the spin-coating process has been used for some time and is fairly simple to employ and practice, it is time consuming, wasteful and inconsistent. Specifically, such processes are especially time consuming with respect to the preparation of the solution, the application of the solution to the substrate, the time needed for allowing the solvent to evaporate and the solution coating to dry on the surface and the subsequent bake operation. Furthermore, while such an operation is suitable for coating small substrates, it is particularly ineffective for providing a uniform coating over a large substrate area. Because of the nature of spin-coating, while coating uniformity tends to be fairly consistent in the center region, as one moves further from the center of the substrate surface, such consistency is lost, particularly in the peripheral regions. Such a result is particularly problematic in large area displays (e.g., European A4 size manufacture) and substrates, therefor, are often deliberately made oversized so that they may subsequently be trimmed following the coating/curing process to cut out that substrate surface having the most uniform coating thickness. As a result, there is a tremendous amount of waste of both the coating solution, as well as the underlying substrate materials. Other shortcomings of the spin-coating process are the possibility of the introduction of contaminants through the solvent to the coating resin itself. Finally, given the continuing trend and movement for environmentally safe and healthy processing, there is a growing desire for elimination of solvents in manufacturing processes.
U.S. Pat. No. 4,038,441 to Dubois describes a method of manufacturing a liquid crystal display device in which an organic polymer deposit is formed upon the internal faces of two electrode-carrying plates by directing onto them, at a very low angle of incidence, a monomer vapor flow. Suitable monomers which have the property of polymerizing from the vapor phase include methyl acrylate, methyl methacrylate, vinyl monomers, silanes, chlorosilanes and siloxanes. The deposition is carried out in a cylindrical enclosure of vitreous silica, the enclosure being surrounded at its central portion by a heater resistor and having a mounting arrangement therein for the substrates. While this process overcomes many of the objections for spin-coating, as mentioned above, it involves a special piece of apparatus and requires high temperatures (e.g., 240.degree. C.) for the deposition of vinyltrichlorosilane.
It is known that cyanoacrylate monomers can be deposited onto a substrate from vapor. U.S. Pat. No. 4,675,273 (Woods et al.), assigned to Loctite (Ireland) Limited, incorporated herein by reference, describes a method for applying a polymeric resist coating to a substrate which comprises exposing the substrate to be coated to the vapor of an anionically polymerizable monomer of the formula I: EQU CHR.dbd.CXY I
wherein X and Y are strong electron withdrawing groups and R is H or, provided that X and Y are both --CN, R may be C.sub.1 -C.sub.4 alkyl, for sufficient time to deposit a polymerizable coating thereon. The monomer condenses and polymerizes on the surface of the substrate to give a highly uniform high molecular weight polymeric coating which is useful as a resist coating in lithographic process. U.S. patent application Ser. No. 07/542,465, filed Jun. 22, 1990, entitled "Photoresists Formed by Polymerization of Di-Unsaturated Monomers," describes a similar method using vapor of a substituted butadiene monomer of the formula II: ##STR2## wherein X and Y are as defined above and R.sup.1 is H or, providing that X and Y are both --CN, R.sup.1 may be aliphatic hydrocarbyl, aryl or alkaryl.
U.S. patent application Ser. No. 07/542,464, filed Jun. 22, 1990, entitled "Anionically Polymerizable Monomers, Polymers Thereof, and Use of Such Polymers in Photoresists," describes anionically polymerizable monomers of the formula III: ##STR3## wherein A.sup.1 is --H or --CH.dbd.CH.sub.2 ; X is as defined above; and Y.sup.1 is a strong electron withdrawing group containing at least one silicon or titanium atom. That Application also describes polymeric resist coatings formed by vapor deposition of a monomer of formula III.
Although cyanoacrylates have found utility as liquid crystal alignment layers, their use as such has not been without shortcomings. Generally, liquid crystal alignment layers formed from cyanoacrylates prepared through the spin-coating process manifest many of the misgivings described above with respect to the spin-coating process in general. Additionally, such liquid crystal alignment layers typically have tilt angles of less than 2.degree.. While vapor deposition of such cyanoacrylate and cyanoacrylate derivative monomers will overcome many of the misgivings of spin-coating, the tilt angle of such liquid crystal alignment layers nevertheless remains less than 2.degree..
It is, therefore, an intent of the present invention to provide a new method for the preparation of liquid crystal alignment layers having high tilt angles. In the preferred embodiment, it is the intent of the present invention to provide such high tilt angle liquid crystal alignment layers without the misgivings of a spin-coating process.